Insects mediate the interactive effects of propagule pressure and resource availability on a plant invasion.
Sanders , Nathan*,1, Weltzin, Jake1, 1 Department of Ecology and Evolutionary Biology, knoxville, TN, United States
ABSTRACT- Three factors are often cited as important controls on plant invasions: resource availability, propagule pressure, and ecological resistance by insect herbivores. However, robust generalities have yet to emerge, perhaps because these factors can interact in complicated ways to affect biological invasions. To examine interactive effects of these three factors on invasion by the exotic plant, Lespedeza cuneata, we initiated a field experiment in an old-field system in 2004. The experiment consists of 72 3 m × 3 m plots. Soil nitrogen (N) availability (at 3 levels), herbivore abundance and diversity (at 2 levels), and propagule pressure of L. cuneata (at 2 levels) were assigned to plots in a fully crossed, completely randomized design, with 6 replicates of each unique treatment combination. Availability of soil N is controlled by additions of either N or carbon (C; added as sucrose), with an unamended control. Propagule pressure treatments include the addition of seeds (at a rate of 1,730 seeds m-2), with an unseeded control. Herbivore diversity and abundance is manipulated at two levels using a combination of insecticide and vacuum sampling. Establishment of seedlings of L. cuneata was controlled by complex interactions among the three experimental variables: At the end of the growing season, the density and biomass of L. cuneata were affected by the propagule and nutrient treatments only in plots that received insecticide. Within plots that received insecticide, the addition of propagules increased the density of L. cuneata in C and control plots, but did not affect density of L. cuneata in N plots. Within the propagule added plots, L. cuneata density was greatest in C plots, intermediate in control plots, and least in N plots. Within plots that received insecticide, biomass of L. cuneata was greater in the propagule-added plots only in the nutrient control plots. Taken together, these results illustrate how both biotic and abiotic factors, acting across multiple trophic levels, mediate invasion dynamics.
Key words: invasion, herbivory, nutrients, propagule
All materials copyright The Ecological Society of America (ESA), and may not be used without written permission.